Some Reasons Not to Use Goldfish to Cycle Your Tank

Tips on Cycling Your New Aquarium

(The First Tank Guide)

I generally don't recommend using goldfish to cycle your
fish tank for many reasons. The most prominent reasons that goldfish
are generally not a good choice for cycling your aquarium are:

Goldfish have different care
requirements than do tropical fish, and if you keep the goldfish
healthy, and then try to adjust the tank for your tropical fish, you
will stress the filter and will cause the
tank to at least partially cycle again - you
could even cause the tank to completely cycle
again.

If you keep the goldfish in tropical
conditions, they will be more susceptible to
disease and are likely to leave you with a diseased tank, treating
the disease will probably kill the filter
and cause the tank to cycle again.

The goldfish people use to cycle a tank are usually feeders. These fish are very poorly cared
for by breeders, shippers, wholesalers, and most retailers, and almost
always bearing some disease, leaving you
with a diseased tank which you have to treat and then cycle again.

And, of course, what are you going to do with the goldfish after you cycle the tank?

This recommendation became common in the 1950s and 1960s when the
pet trade and aquarium hobbyists were just beginning to understand the
cycling process. Since then much has been learned about how the
process actually works, and though using a large number of fish - or
especially using a large number of feeder goldfish - to cycle your
aquarium had fallen out of favor with educated aquarium experts,
humane aquarium enthusiasts, and pet shops
that really wanted to provide quality information to their customers
and help them through the cycling process by the early 1980s.

I really have no idea why information that has been known to be
inaccurate and even cruel to animals is still so widely recommended
over three decades since it was known to be bad - or even harmful -
information is still so prevalent, but among the uneducated masses it
is still prevalent, and even some pet shops that claim to be humane,
quality, or expert train their employees to tell customers to cycle
their fish tank with a large number of feeder goldfish...

Of course, if you are planning to keep goldfish in the tank, then
several of these considerations become moot. For example, if you are
planning to keep goldfish, then you know what you are going to do with
the goldfish once the tank cycles: you'll keep them. You'll also be
using goldfish that you want to keep, not feeder goldfish, so you'll be using
healthier, more robust, less disease ridden fish as well, improving
your changes of success. and finally, you'll be keeping your goldfish
in water conditions appropriate to goldfish.